ISOFLUXES: Direkt eddy covariance measurements of the stable isotope composition of CO2 and H2O fluxes between forest and atmosphere
Final Report Abstract
The use of stable isotopes in carbon dioxide (CO2) and water vapor (H2Ov) cycles helped substantially to investigate the CO2 and H2Ov exchange between terrestrial ecosystems and the atmosphere. Over the last decades, the potential of stable isotope-based methods was enhanced by recent developments of laser-based optical spectrometers, in particular regarding field measurements on ecosystem scale. Within the framework of this research project, we used two laser-based analyzers in combination with two methods to investigate fluxes on ecosystem scale to investigate isotopic compositions of CO2 and H2O fluxes in a managed beech forest. We measured the δ18O and δD composition of evapotranspiration (ET) with the eddy covariance (EC) method. Additionally, we measured the δ18O and δ13C composition of nighttime CO2 exchange (respiration). Further, we obtained a data set of EC measurements for the stable isotope composition of CO2 net fluxes for future data evaluation. This dataset is planned to be combined with the EC measurements of δET presented here to provide further insights into 18O exchange between CO2 and H2O. As the two laser-based analyzers used for these measurements were new (or highly modified, which was the case for the water vapor analyzer), rigorous instrument tests were necessary. Thus, we presented instrument characteristics such as an evaluation of the calibration strategy, and measurements of precision and potential accuracy for both analyzers. Based on these measurements, we conclude that if an intense calibration strategy is applied, the used analyzers can compete with other more often used optical spectrometers. In combination with the Keeling plot method (for CO2) and the eddy covariance method (for H2Ov), we could use these analyzers to measure the seasonal variability of the respective flux or flux component. We further estimated the uncertainties of the respective measurements. The uncertainty estimate for the EC measurements δET were 0.5‰ for δ18O and 1‰ for δD on hourly timescale. The uncertainty of the Keeling plot intercept measurements of nighttime CO2 exchange was 0.8‰ for both δ18O as well as for δ13C on a 90 minute timescale. In the case of the EC measurement of δET, we obtained the first time series of the isotopic composition of water vapor fluxes above a forest ecosystem measured with EC. As the used analyzer is currently the only fast (>2Hz) analyzer that is currently commercially available, the applicability of this analyzer for EC measurements could be the basis for various future studies to investigate the water cycle above tall ecosystems with the EC method. The obtained time series of δ18OET and δDET provide deeper insights into the underlying processes. In the case of δET, a plot of the measured isotopic compositions in the in the δ18O-δD-plane allows to separate the transpiration-dominated from the evaporation-dominated periods. Further, we used the measured isotopic composition to analyze the influence of local ET on the isotopic composition of ambient water vapor δv. Here, we did not find any evidence for a dominant control of δv by local ET, neither on sub diurnal nor on day-to-day scale, even if δv was measured close to the canopy. Regarding respiration, the time series of δ18O of nighttime respiration reveals a sharp decrease of about 30‰ within 10 days subsequent to a short and early snow event. To evaluate the seasonal variability of δ13C of nighttime CO2 exchange, we correlated the measured isotopic composition with its potential meteorological drivers. For the time period before the first snow, when the ecosystem acted as a net CO2 sink, we found that the δ13C composition of respiration is linked to time-lagged net radiation. The time lag of this correlation is consistent to the time lag between assimilation and respiration. By comparison with modeled data, we further found that the sign of this correlation is not consistent with an explanation of this correlation by 13C discrimination. In summary, we conclude that the conducted lased-based measurements of the dual isotope composition of H2Ov and CO2 fluxes or flux components are feasible and can provide deeper insights into the underlying exchange processes. In particular, dual isotope measurements might have a great potential to analyze biosphere-atmosphere exchange processes. In future, a combination of EC measurements of δ18O in CO2 and H2Ov fluxes seems a) feasible and b) promising.
Publications
- (2017) A new instrument for stable isotope measurements of 13C and 18O in CO2 – instrument performance and ecological application of the Delta Ray IRIS analyzer, Atmospheric Measurement Techniques, 10, 4537–4560
Braden-Behrens, Jelka; Yan, Yuan; Knohl, Alexander
(See online at https://doi.org/10.5194/amt-10-4537-2017) - (2017) Measuring the isotopic composition of ecosystem respiration in a temperate beech forest, Thermo Scientific White Paper, WP30385
Braden-Behrens, Jelka; Knohl, Alexander; Jost, Hans-Jürg; Mandic, Magda
- (2018) The isotopic composition of CO2 and H2Ov fluxes in a managed beech forest - Instrument tests and ecological application of two laser-based absorption spectrometers, PhD thesis
Braden-Behrens, Jelka
